Field of the Invention
The present invention relates to a soft magnetic component for a torque sensor used for detection of a torque and an angular velocity utilizing rotation of an axis, particularly, a soft magnetic component for a torque sensor used in a ring core and a yoke, and a torque sensor using the same, particularly, a torque sensor used in an electric power steering (hereinafter abbreviated to “EPS”).
Description of the Background Art
The EPS is a system that properly assists a steering force by detecting the steering force from a steering wheel with a torque sensor, transmitting the detection signal as a torque signal to a control unit, and performing control of applying, to a motor, an electric current according to the torque signal. This torque sensor used in the EPS includes an input shaft coupled to the steering wheel, an output shaft coupled to a steering mechanism, a torsion bar coupling these two shafts, north poles and south poles of magnets provided around, this torsion bar, a yoke, a ring core, and a detection portion including a Hall element or the like that detects magnetic flux, for example. For example, the yoke and the ring core may be resin-molded.
In this torque sensor, application of the steering force leads to rotation of the input shaft and torsion of the torsion bar to cause angular differences between the magnets and the yoke. According to the angular differences, the magnetic flux of the magnets are transmitted from the yoke to the ring core, and are detected by the detection portion as magnetic flux proportional to the torsional angle of the torsion bar. Thus, a steering torque according to the steering force can be detected.
In general, when an external magnetic field (magnetic field) is applied, to a magnetic body, alternately also including an opposite direction, a magnetization curve (B-H curve) represented by a magnetic flux density (B) and the intensity of an external magnetic field (H) is a hysteresis curve. The gradient of the hysteresis curve is called a magnetic permeability μ (=B/H, [H/m] in International System of Units), the gradient in the vicinity of an original point is called an initial magnetic permeability μi, and the maximum gradient is called a maximum magnetic permeability μm. As a magnetic permeability, a ratio (relative magnetic permeability) of the magnetic permeability to a vacuum magnetic permeability (μ0=4π×10−7 [H/m]) is generally used, and hence this is hereinafter followed. A magnetic body having a larger relative magnetic permeability described above is more likely to be magnetized by a weak magnetic field. Thus, in order to increase the sensitivity of the torque sensor, a magnetic body more greatly magnetized by as a weak magnetic field as possible is preferably used. More specifically, a magnetic body having a larger relative magnetic permeability is preferably used. Therefore, a coercive force (Hc) indicating the intensity of an external magnetic field necessary to reduce the magnetic flux density of the magnetic body to zero is preferably as small as possible.
For the ring core and yoke used in the torque sensor, a material defined as SUS410L, a Fe-based material having a small strain sensitivity disclosed in Patent Document 1 (Japanese Patent Application No. 2003-309007), a Fe—Ni based material having a large relative magnetic permeability and a small core loss disclosed in Patent Document 2 (Japanese Patent Application No. S63-68982), a Fe—Ni based material defined in JIS-C2531, or the like is used, for example. Particularly, permalloy C (PC), which is a Fe—Ni based material, defined in JIS-C2531, having an initial relative magnetic permeability and a maximum relative magnetic permeability increased by adding about 78 mass % of Ni and Fe and further adding Mo, Cu, Cr, etc. is excellent in sensitivity to a change in magnetic flux, and hence it is effective for improving the detection accuracy.
In recent years, for a torque sensor, resin molding has been increasingly applied to molding of a member containing a soft, magnetic material in order to reduce the number of components, reduce the man-hours, and reduce the weight, for example. Generally, when the soft magnetic material is resin-molded, a compression stress is generated inside the soft magnetic material by resin shrinkage after resin molding, the initial relative magnetic permeability, the maximum relative magnetic permeability, and the effective relative magnetic permeability are reduced as compared with before resin molding, and the coercive force is increased. In other words, the magnetic properties of a soft magnetic component containing the soft, magnetic material are degraded by resin molding. Thus, there is such a problem that the torque detection accuracy of a torque sensor using the resin-molded soft magnetic component for a torque sensor is reduced as compared with that of conventional ones by an increase in hysteresis and reductions in responsiveness and sensitivity.